DVD ROM, Hard drive and NIC
The boot sequence order of devices is the order in which your computer will boot its devices. You can usually choose from the hard-drive, the disk-drive, and a few more options.
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How many additional devices can be assigned in the boot
The BIOS recognizes the boot device during the Power-On Self-Test (POST) process by scanning the configured boot order in its settings. It looks for bootable devices, such as hard drives, SSDs, USB drives, or optical drives, and checks for a valid boot sector on those devices. Once it identifies a bootable device, the BIOS transfers control to that device's bootloader to initiate the operating system startup process. If no bootable device is found, it typically displays an error message or prompts the user to check connections.
BIOS setup
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A computer does not need to have any input or output devices plugged into it at all in order to boot, as long as it is configured to not stop on missing devices; the most common "stop error" for missing devices is a keyboard, and this can typically be disabled.
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The message "No bootable devices" typically indicates that a computer's BIOS or UEFI firmware cannot find a device with a valid operating system to boot from. This can occur due to various reasons, such as a disconnected or malfunctioning hard drive, incorrect boot order settings, or corrupted operating system files. Users may need to check hardware connections, verify BIOS settings, or repair/reinstall the operating system to resolve the issue.
BIOS is looking for a boot sector. If the disk does not have a valid boot sector at the address which is also known by the BIOS, it won't boot from disk.
Post
The SCSI logical unit number for boot devices is almost always zero(0).Matt Friend